The study shows that delivery of a microbicide from a vaginal ring can be effective, and suggests potential for the success of such rings in women, according to the researchers at the Population Council, an international nonprofit group.

However, research conducted with animals doesn't always produce the same results in humans.

Microbicides are substances that can be applied inside the vagina or rectum to protect against HIV, which is the AIDS-causing virus, and other sexually transmitted infections.

In this study, council scientists placed vaginal rings treated with either a microbicide or a placebo in macaque monkeys. The animals were then exposed to a single dose of SHIV, a virus that combines genes from HIV and SIV, the monkey version of HIV.

Two of 17 macaques with the microbicide vaginal rings became infected, compared with 11 of 16 macaques with the placebo vaginal rings. That means that the microbicide vaginal rings were 83 percent effective in protecting against the virus.

"This proof-of-concept study confirms that the investment in vaginal rings as a delivery system for HIV prevention is paying off," Naomi Rutenberg, vice president and director of the Population Council's HIV and AIDS Program, said in a council news release. "Our findings show that rings can deliver an anti-HIV drug to prevent infection."

An effective vaginal ring with microbicides would solve a common problem with microbicides in gel form, which is the risk that users will forget to use them or apply them improperly.

The Population Council wants to develop a vaginal ring that women can insert and leave in place for up to three months. Current models, used to prevent pregnancy, are treated with the hormones estrogen and progestin and placed in the vagina once a month.